1. Field of the Invention
The present invention relates to exposure apparatuses, exposure methods and device manufacturing methods, and more particularly, to an exposure apparatus and an exposure method used in a lithography process for manufacturing microdevices such as semiconductor devices, and a device manufacturing method using the exposure method.
2. Description of the Background Art
Conventionally, in a lithography process for manufacturing electron devices (microdevices) such as semiconductor devices (such as integrated circuits) and liquid crystal display devices, an exposure apparatus such as a projection exposure apparatus by a step-and-repeat method (a so-called stepper), or a projection exposure apparatus by a step-and-scan method (a so-called scanning stepper (which is also called a scanner)) is mainly used.
In this type of the exposure apparatus, in order to improve the resolution of a projection optical system, the wavelength of exposure light has been shortened and the numerical aperture of the projection optical system has been increased (increased NA). However, due to the shortened wavelength of exposure light and the increased NA of the projection optical system, a depth of focus has been narrowed. Therefore, as a method to substantially shorten the exposure wavelength, and also increase (widen) a depth of focus compared with in the air, an exposure apparatus making use of a liquid immersion method has been recently gathering attention.
Meanwhile, as well as the high resolution, high throughput is also required for an exposure apparatus. As a method to improve the throughput, various types of a twin-wafer-stage type exposure apparatus are proposed that employ a method in which a plurality of wafer stages to hold a wafer, e.g., two wafer stages are provided, and different operations are concurrently processed on the two wafer stages.
Moreover, recently, a twin-wafer-stage type exposure apparatus that employs a liquid immersion exposure method has been also proposed (e.g., refer to U.S. Pat. No. 7,161,659).
In the meantime, in the case of measuring the position (height) of a wafer stage in a Z-axis direction that is orthogonal to an XY plane, a Z interferometer is known that measures the height of the wafer stage by irradiating a measurement beam parallel to the XY plane to a reflection surface (a reflection surface for Z measurement) that is placed on a side surface of the wafer stage so as to be inclined at a predetermined angle, e.g., an angle of 45 degrees with respect to the XY plane, and receiving a return light of the beam (e.g., refer to U.S. Pat. No. 6,208,407).
However, in the case where the above-described Z interferometer is employed in an exposure apparatus of a similar type to the exposure apparatus disclosed in U.S. Pat. No. 7,161,659, a configuration is desired in which height measurement of both the wafer stages can be performed with the Z interferometers on both sides. With such a configuration, however, when the two stages were made to be in contact or in proximity and a liquid immersion area (a liquid) was delivered between both the wafer stages as is disclosed in U.S. Pat. No. 6,208,407, there was a possibility that the reflection surfaces for Z measurement came into contact with each other and had damages. Further, in order to prevent such a situation from arising, when trying to perform the delivery of the liquid immersion area in a state where the two wafer stages were apart from each other, there was a possibility that the liquid leaked to between both the wafer stages and the delivery of the liquid immersion area could not be performed. In addition, the leaked liquid could wet the reflection surfaces for Z measurement. Further, even if the reflection surfaces for Z measurement are not used, in the cases such as where at least one of the wafer stages has a mechanism section that protrudes beyond other sections, the problem similar to the above-described one could occur when the liquid immersion area (the liquid) is delivered between the two wafer stages with both the wafer stages being in contact or in proximity.